Delete instaces tmp diectory on startup

[Trial97]

The .tmp folder that is inside the instance folder should not exist or be empty.
This ensures that the mentioned folder will be deleted on launch

[TayouVR]

is this one used when downloading or updating instances?
Either way, cleaner would be if we used the system temp, or QTemporaryFile or smth.

But that doesn't really matter here, clearing it on start makes sense ^^

[Trial97]

Yes it's the folder we use for update/create instances.
No we can't use the qtemp folder as that would require to add another step in the instance creation/update(copy the files from temp to instances). Right now due to files being on same partition we can just rename with 0 cost.